The folate receptor (FR) type a is overexpressed in major types of gynecological malignancies, particularly in ovarian, uterine and cervical cancers whereas its expression in normal tissues is restricted to the luminal surface of certain epithelial cells where it is inaccessible to circulation. Consequently, FR-alpha is a promising tumor target for a variety of therapeutic approaches, currently in pre-clinical studies and in clinical trials, and for tumor imaging agents, which are expected to be introduced soon for clinical use. In searching for ways to specifically modulate FR-alpha in tumors we have found that in HeLa (cervical carcinoma) cells and in BG-1 (ovarian carcinoma) cells, in the presence of subnanomolar concentrations of estrogen, the estrogen receptor (ER) rapidly represses the FR-alpha gene promoter and strikingly downregulates endogenous FR-alpha expression; the estrogen effect is rapidly reversed by tamoxifen and ICI 182,780. We hypothesize that in vivo, in ER-positive and FR-alpha positive gynecological tumors, FR- alpha expression is repressed by circulating estrogen and that brief treatment with antiestrogens will produce a substantial increase in FR- alpha expression by derepression of the FR-alpha promoter. Based on observations with a mutant form of ER, we further predict that certain ER ligands will not only derepress the FR-alpha promoter but will further activate it. Since modulation of the FR-alpha gene in this manner may be expected to considerably enhance the sensitivity of tumor imaging and the efficacy of FR-targeted therapies, we request funding to establish a strong experimental basis to warrant the use of ER ligands in clinical studies of FR-alpha targeting. Accordingly, the main goals of this proposal are to: (i) understand the mechanism of transcriptional modulation of the FR-alpha gene by ER ligands and (ii) examine the potential for antiestrogens to upregulate FR-alpha in tumors in vivo by developing several ER and FR-alpha expressing mouse tumor xenograft models. This study should, in addition, help ongoing efforts in this laboratory to isolate critical regulatory elements in FR genes to develop gene therapy vectors for tumor-specific expression of exogenous genes and may also uncover novel mediators of gene repression by estrogen.